Letters, Correspondences by Andrew K Fletcher
New Scientist Journal Excuses "Another Brick Wall"
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Andrew Fletcher 26 Berry Drive Paignton Devon TQ3 3QW
18 April 1995
First let me apologise for having taken so long to write to you about your ideas on how solute concentration gradients could drive fluid flow. Working on special projects, as we have been for the past few weeks, often makes it difficult for us to deal with other suggestions as quickly as we would like to.
In this case, the delay is especially unfortunate as, having now had time to think about your ideas in the light of the comments by Dr Cutler, I'm not persuaded they have a strong claim on our space at this stage. Let me explain why.
As I see it, your core idea is that scientists have overlooked one of the most important mechanisms driving fluid flow in trees and plants—namely, the effect that concentrated phloem solutions at the tops of plants have on more dilute fluids at the roots (the downward force of one causing the other to be sucked up). The problem for us is that the picture cannot really be this simple. As Dr Cutler points out, sap in phloem tends to move in the direction of demand, laterally as well as vertically: what happens to the fluid flow system when the concentrated solutions are all in the bottom half of the tree? If the downward force of the "heavy" solution was the main thing then presumably the tree would be in trouble.
But we know trees don't (normally) run into this kind of trouble, which suggests that even if there is a contribution to fluid flow from the phenomenon you describe, it must be less important than capillary action and other forms of root pressure.
As to the wider implications of the phenomenon, I'm afraid we cannot see a strong case for giving them publicity in the absence of good correlative evidence, though we appreciate that your intention at this stage is merely to air them in a speculative fashion.
I'm sorry we can't be more positive, particularly in light of the delay. I am returning the copy of the video and tape.
Life Sciences Editor
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